1. Field of the Invention
The present invention relates to a stepping motor driven through two-phase excitation for use in a disk drive device or the like and a carriage transport mechanism employing such a stepping motor.
2. Description of Related Art
FIG. 6 is a sectional view showing a basic structure of a two-phase stepping motor.
Referring to FIG. 6, a rotor 1 is integrally constructed of a cylindrical magnet 2, magnetized such that poles N and poles S are alternately formed in its circumferential direction, and a shaft 3 arranged along the central axis of the magnet 2. Both end portions of the shaft 3 are supported for rotation by bearings, not shown. There are provided a first stator 4 and a second stator 5 both being ring-shaped and having the rotor 1 inserted in the central cavity portion thereof with a gap therebetween. The bottom face of the first stator 4 and the top face of the second stator 5 are fixedly joined together. The first stator 4 is formed of a combination of an upper core 6 and a lower core 7 with a high permeability characteristic and an annular exciting coil 8 arranged between both the cores 6 and 7. On the inner circumferential portion of the upper core 6, there are provided pole teeth 6a protruding downward in the form of a comb, and on the inner circumferential portion of the lower core 7, there are provided pole teeth (not shown) protruding upward in the form of a comb, these plural pole teeth being interleaved with each other and confronting the outer circumferential face (magnetized face) of the magnet 2. Similarly, the second stator 5 is formed of a combination of an upper core 9, with pole teeth (not shown) protruding downward in the form of a comb on its inner circumferential portion, and a lower core 10 with pole teeth 10a protruding upward in the form of a comb on its inner circumferential portion, and an annular exciting coil 11 arranged between both the cores 9 and 10. As described later, the pole teeth of the first stator 4 are offset in the circumferential direction from the pole teeth of the second stator 5, whereby there is usually provided a phase difference of 90 degrees in electrical angle between both the stators 4 and 5.
The stepping motor arranged as described above is adapted such that the rotor 1 is rotated by an angle corresponding to the number of steps when the exciting coils 8 and 11 are supplied with a pulse voltage and, thereby, the pole teeth of the cores 6, 7, 9, and 10 of the first and second stators 4 and 5 are excited.
FIG. 7 is a development of pole teeth of a conventional stepping motor of the described type, in which the pole teeth 6a and 7a of the upper and lower cores 6 and 7 of the first stator 4 are interleaved with a pitch .alpha. evenly dividing the total circumference. Also, the pole teeth 9a and 10a of the upper and lower cores 9 and 10 of the second stator 5 are interleaved with the pitch .alpha., but the pole teeth of the first stator 4 (the phase thereof will be called phase A) and the pole teeth of the second stator 5 (the phase thereof will be called phase B) are arranged to be offset from each other by .theta.=.alpha./2 in the circumferential direction. Here, since the angle .alpha. corresponds to an electrical angle of 180 degrees, the phase difference .theta. between the phase A and the phase B corresponds to 90 degrees in electrical angle.
The pattern of excitation of the above described two-phase stepping motor is as shown in FIG. 12. A vector diagram of the generated torques is shown in FIG. 8. The combined torques A+B and A-B respectively vary with the angle of rotation as shown in FIG. 9 and FIG. 10.
Accordingly, in the above described stepping motor, the combined torques A+B and A-B become the same in absolute value (refer to FIG. 9 and FIG. 10) and the torque generated at the time of stepping becomes as shown in FIG. 11.
In a two-phase-excited operation of such a stepping motor as described above, when the number of steps used for setting up each motion is an even number at all times, a higher starting speed can be obtained even with a large load, and following startup, a shorter step rate can be achieved by increasing the torque at the time of startup (odd-numbered step), and reduction of the overshoot and shortening of the settling time can be achieved by decreasing the torque at the time of settling (even-numbered step). However, the torques at the time of startup and settling in the above described conventional motor are of the same magnitude, i.e., the generated torque is not changed with the number of steps, and therefore, there has been such a difficulty that if it is attempted to increase the starting torque, the settling torque is also increased, and conversely, if it is attempted to decrease the latter, the former is also decreased. Further, high speed feeding has been unachievable by a carriage transport device employing such a stepping motor.